1. Field of the Invention
This invention relates to an electron gun structure and particularly to the improvement of an electron gun used for a color cathode ray tube.
2. Description of the Prior Art
Generally, in an electron gun for a cathode ray tube an electron current emitted from a cathode is controlled by a plurality of grid electrodes and radiated as a sharp electron beam of high energy.
In one type of electron gun used for a color picture tube, the various electrodes are mounted on a plurality of glass support rods. These electrodes include three cathode electrodes and five grid electrodes. Each cathode electrode comprises a tubular cathode sleeve with an oxide of electron emissive material being applied as a coating on the forward end surface of the sleeve. Each cathode is indirectly heated by a heater coil which is positioned within the tubular sleeve and which has legs welded to heater supporting straps and mounted by studs on the rods.
In a color picture tube, a high voltage of 20kV to 30kV is applied between this electron gun and the anode of the tube, and this anode voltage may be charged on the inside wall of the neck bulb of the tube and directly discharged from the anode to the grid electrodes or the cathode electrode.
As a result, a high voltage is induced between the first grid electrode and the cathode electrode. This voltage can produce arcing between the cathode and the first grid electrode. Moreover, arcing may also occur between the open edge of the cathode sleeve of the cathode electrode and the facing part of the heater coil.
When arcing occurs between the cathode sleeve and the heater coil, the insulating layer (generally alumina sintered material) on the surface of the heater coil deteriorates so that either the heater and the cathode sleeve of the cathode electrode are short-circuited or a discharge current flows in the heater so that the heater fuses.
Therefore, conventionally, an insulator such as a ceramic disc or a spacer coil has been interposed between the heater and the cathode sleeve. But when an insulator is inserted between the heater and the cathode sleeve, the heat conduction from the heater to the cathode sleeve becomes poor. Also, raising the electron emissive surface to the required temperature takes an excessively long time. Therefore this kind of spacer cannot be used in an electron gun of the so called quick emission start type. Recently, moreover, there has been a strong tendency to lower the power consumed by the heater, in order to save energy.
In order to lower the heater power, it is necessary to make the core wire of the heater thin and long. But the extent to which it can be lengthened is limited because of the capacity of the cathode sleeve and accordingly the core wire must necessarily be made thinner. However, when the core wire is made thinner, breakage of the wire takes place more readily when arcing to the heater has occured.
Also, in recent years, the first grid electrode has commonly been made in the form of a plate. In an electron gun having such a plate type first grid electrode, electrons leak from the first grid electrode to the cathode electrode, and because of the high voltage induced in the glass support rods and the inside wall of the neck bulb of the tube, arcing between the glass rod and the neck bulb occurs. Also, evaporated metal sputtered from the electron emissive coating adheres to the glass rods and causes insulation breakdown between a cathode and the first grid electrode.
In order to prevent these occurrances, the bead is arranged in the first grid electrode so as to surround the electron emissive coating as much as possible, but even then fouling of the glass rods by sputter cannot be prevented altogether.